Many applications, such as automotive and aircraft fabrication, require precisely positioned holes to be drilled in a workpiece. The holes must be accurately aligned with only relatively small positioning errors tolerable. Accordingly, a drill plate is typically aligned and mounted on the workpiece prior to drilling holes through the workpiece. The drill plate has accurately positioned holes extending therethrough such that by properly aligning the drill plate with respect to the workpiece, each hole in the drill plate precisely overlies the desired location for the corresponding hole to be drilled in the workpiece.
In many instances, it is desirable to attach the drill motor to the drill plate such that the operator does not have to offset the full weight of the drill motor and the thrust generated during the drilling operation. Attachment of the drill motor to the drill plate further ensures proper alignment of the cutting element with the desired hole location in the workpiece. In many instances, the accurately positioned holes must be drilled in relatively hard materials, such as titanium. To drill holes in such materials, relatively great axial pressure must be applied to force the cutting element into and through the material. Accordingly, the affixation of the drill motor to a drill plate assists the operator in applying sufficient axial pressure to the cutting element.
Drill motors are typically attached to a drill plate by means of lock screws and a specially designed drill bushing. For example, a pair of lock screws may be positioned on opposite sides of an aperture defined in a drill plate. Lock screws typically have an unthreaded portion underlying the slotted head for receiving a corresponding flange extending outwardly from the drill bushing. Accordingly, the drill bushing may be oriented between the lock screws and rotated such that the outwardly extending flange of the drill bushing is engaged and retained beneath the slotted head of the lock screws. The bushing is thus affixed to the front surface of the drill plate.
The use of lock screws and specially designed drill bushings, however, requires the lock screws to be accurately positioned on the drill plate. In addition, the use of lock screws prevents the incorporation of immediately adjacent apertures in the drill plate since lock screws must be installed on each side of the apertures. In addition, lock screws may require periodic replacement to maintain accurate alignment with the aperture in the drill plate.
In order to securely clamp a drill motor to a drill plate without the use of lock screws, specially adapted sleeves and bushings have been designed for incorporation into the drill plate. Examples of such sleeves and bushings incorporated in drill plates include those described in U.S. Pat. No. 5,161,923 (the '923 patent) to Helmut Reccius on Nov. 10, 1992 and U.S. Pat. No. 4,740,117 (the '117 patent) to Marie-Jose Schaff Deleury et al. on Apr. 26, 1988. The drill plate described in the '923 patent includes a cylindrical protrusion extending outwardly from the front face of the drill plate and a recessed periphery about the aperture on the rear face of the drill plate to align and support the drill motor. The machine tool of the '923 patent includes a nosepiece having a number of longitudinal tongues. The tongues are adapted to expand within the aperture of the drill plate and be received within the recessed periphery of the aperture.
Further, the '117 patent discloses drill bushing inserts or sleeves within apertures defined in the drill plate. The inserts have a cylindrical portion extending outwardly from the front face of the drill plate about each aperture. The cylindrical portion has a flanged end for mating with the correspondingly designed drill motor. In particular, the drill motor has a plurality of protruding fingers having a flanged end portion adapted to mate with the flanged end of the cylindrical portion.
In addition, U.S. Pat. No. 2,839,953 (the '953 patent) to Randolph T. Hanger on Jun. 24, 1958 also incorporates a sleeve within the aperture defined in the drill plate. The drill bushing of the '953 patent is manually expanded within the tapered sleeve to attach the drill motor to the drill plate.
The fabrication of specially designed drill plates, however, increases the cost of the drill plates and the time required for their manufacture. The use of sleeves or bushings within the apertures of a drill plate also prevents the incorporation of overlapping apertures in the drill plate. In addition, such specially designed drill plates may generally only be used with drill motors having bushings designed to mate with the corresponding sleeves or bushings of the drill plate.
Drill bushings have also been designed for attachment to drill motors in order to clamp the drill motor to a drill plate. For example, U.S. Pat. No. 2,935,905 (the '905 patent) to J. C. Winslow on May 10, 1960 discloses a drill bushing adapted for extension through an aperture in a drill plate and for attachment thereto. The drill bushing of the '905 patent incorporates a pneumatically-actuated ram assembly for extending the drill bushing through the aperture in the drill plate. The ram assembly, however, is offset from the cutting element. Accordingly, the drill bushing of the '905 patent may not readily access all areas with limited lateral access.
Thus, while it would be desirable to have a drill bushing for securely clamping a drill motor to a drill plate in order to drill precisely aligned holes, drill bushings still suffer from a number of deficiencies, including the need for specially designed bushings or sleeves in the drill plate or the inability to reach areas of limited lateral access. In particular, it would be desirable to have a drill bushing which securely clamps a drill motor to a drill plate without requiring a bushing or sleeve to be incorporated in the drill plate and which is controlled with a pneumatically-actuated piston concentrically aligned with the cutting element.